Alphaviruses are important causes of mosquito-borne encephalitis. Sindbis virus provides a murine model for the study of alphaviral encephalitis. Strains of Sindbis virus vary in virulence for mice of different ages as a consequence of defined amino acid differences in the El and E2 surface glycoproteins and undefined changes in the nonstructural proteins and nontranslated regions of the genome. We have shown that a Gly to Arg change in E2-172 is associated with a decreased ability of the virus to bind to neuronal cells leading to a delay in virus replication in the nervous system. A major focus of this proposal is to define and to identify the mechanisms for differences in virulence related to other virus coding changes using recombinant viruses and to identify any contribution of the host using mice of different ages and strains. Specific aims of this proposal are: (1) To determine the importance of amino acid substitutions at positions 3, 55 and 172 of the E2 glycoprotein for target cell binding, virus entry and virulence in mice. (2) To determine the effect of amino acid changes at positions 72, 75, 237 and 313 of the El glycoprotein for membrane fusion, virus entry and virulence. (3) To determine the contribution of the host immune response to neurovirulence in immunologically mature mice. (4) To determine the effects changes in the nonstructural region and of mutations in the 5' and 3' nontranslated regions on virulence. (5) To determine the relative contributions of structural and nonstructural protein coding regions of Sindbis and Ross River viruses to the development of encephalitis versus myositis in mice.